About 500 patients are affected by the evacuation of the city hospital, which is located near the East River in Manhattan but not in the worst part of Manhattan's flooded areas.

Several area hospitals have been contacted about the evacuation and are coordinating efforts to take on some Bellevue patients, including The Mount Sinai Hospital and St Luke's Roosevelt Hospital Centre.

New York mayor Michael Bloomberg says the evacuation is needed because of damage sustained during Sandy.

"They didn't think the damage was that bad and they had a generator going," he told a press conference.

"But the bottom line is when they got into the basement, they realised there was more damage.

"This is going to affect something like 500 patients."

A long line of ambulances was waiting outside the hospital to ferry patients to other medical centres.

Jarron Franklyn, 28, who works in Bellevue's rehabilitation department, says a back-up generator is still running.

"The power is down and we have flooding in the basement," he said.

Dennis Jiosne, 34, a patient from Point Pleasant New Jersey, was evacuated by stairs from the hospital's 16th floor.

He says there were National Guardsmen in the stair well passing people food and water.

Mr Jiosne, who was being treated for a septic ulcer, says the power went out two days ago and that there had been no running water in his room.

But he appears to be taking it all in stride.

"In my unit it really wasn't bad, aside from the plumbing and the food," he said.

"I'm a pretty resilient guy. I was content in my room.

"The lack of television was an inconvenience."

A handful of other New York hospitals have already been evacuated due to the storm that caused record flooding around the city.

New York University's Langone Medical Centre near the city's East River was previously forced to evacuate all 215 of its patients, including critically-ill infants, when its back-up generator failed after some three metres of water flooded its basement.

The Manhattan Veteran Affairs Hospital and the New York Downtown Hospital, both in low-lying areas of lower Manhattan, evacuated patients before the storm hit, and Brooklyn's Coney Island Hospital near the Atlantic Ocean beaches was later evacuated.